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Practical approach and problems in in-situ RGA “calibration” … · Accelerator Science and...
Transcript of Practical approach and problems in in-situ RGA “calibration” … · Accelerator Science and...
Practical approach and problems in
in-situ RGA “calibration”
Oleg Malyshev and Keith Middleman
Vacuum Science Group,
ASTeC –
Accelerator Science and Technology Centre
STFC Daresbury Laboratory
UK
1 Workshop on measurement characteristics and use of quadrupole mass spectrometers for vacuum applications,
EMRP IND12. Bled, Slovenia, April 10–13, 2012.
ASTeC Vacuum Science group main interests
Vacuum in particle accelerators: Achieving, measuring, modelling, designing...
Vacuum related studies in the VS lab: Thermal outgassing
Gauge and RGA calibration
Pumping property measurements
Electron stimulated desorption
Surface coatings and analysis
Photocathode development
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What RGAs used
Make: MKS (Microvision) – 8
Hiden – 1
SX200 (head) + VGQ – 15
Dyson – 1
Prisma – 3
Modes used: FAR and SEM
Profile, trend and MID
Leak detection
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Total pressure range: from 10-5 down to below
10-12 mbar
Other requirements: Bakeability,
Stability, including XHV
Traceability
Low outgassing
A wish: No setting change after
initial calibration check and necessary adjustments
Why we need the RGA calibration
A need of quantitative partial pressure measurements Lack of space for two instruments: gauge + RGA
Outgassing of „gauge + RGA‟ is greater than RGA only
„Strange‟ experimental results received with RGA output data Example: sticking probability >1 ?!
Pimping speed S > Sideal = A v/4
It does not take too long to find that: RGAs are not calibrated – they have a number of factory set
parameters I(m/e=28) usually correspond to P(N2) measured with a UHV gauge
RGAs are adjusted with injection of noble gases – not for residual gases in UHV
Generally, „as-received‟ it is a qualitative (not a quantitative) device
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Definitions of RGA calibration
(1)
Peak alignment
Width alignment
Use of noble gas
mixture
(2)
Accurate partial pressure
measurement
Different type of gases
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Questions:
Influence of cracking pattern
Influence of RGA‟s “gas factory”
An influence of a large peak on a neighbour
small peaks
Influence of ESD in the ion source
Set-up for NEG pumping evaluation
Test chamber 1
(option)
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O.B. Malyshev and K.J. Middleman.
In situ ultrahigh vacuum residual gas
analyzer “calibration”.
J. Vac. Sci. Technol. A 26 (2008), p. 1474.
Main steps of in-situ „calibration‟ on the research rig
Choice of injected gases
Cleaning of injected gases (if necessary)
Filling the gas chamber with known volume and high
accuracy Baratron gauge
If necessary, checking the Extractor gauge calibration vs
High Accuracy Baratron gauge
FS=1.3 mbar; Res = 10-6 mbar
Calibrating RGA vs Extractor gauge by gas injection
Analysis of calibration data to obtain calibration
coefficients.
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Injected gases
H2, CH4, CO, CO2, same as in the
residual gas spectrum,
N2, Ar, O2
LN2 trap for cleaning of injected
gases
Very useful to reduce an impurity of
injected gases (even for class 9999
gases)
H2O is present for calibration
before a bakeout or by heating a
small part of vacuum chamber, or
switching on a filament
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Checking the extractor gauge ex-situ
The extractor gauge is calibrated on the secondary calibration facility in ASTeC Vacuum Laboratory against two primary calibrated (at PTB and NPL) extractor gauges
Advantage: accurate calibration to the secondary standard
Disadvantage: requires a lot of work: removing the gauge from an experimental
installation,
installing it on the calibration facility,
perform bakeout,
calibration,
transfer it back to the experimental installation
Traceability against N2 only...
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Checking the extractor gauge calibration in-situ
If there are any doubt about the gauge error in factor 2 or more:
Gas expansion from the 0.15-l chamber initially filled with N2 at about 10-3 mbar to the 1.5-l chamber
Pumping gas out from the 1.5-l chamber
Expansion to the 1.5-l chamber repeated.
Pressure at 0.15-l chamber is a few 10-6 mbar (not accurately measurable by Baratron gauge but well calculated).
Expansion to the test chamber (valve to the pumps closed). Equilibrium pressure is in the range of 10-7 mbar
Comparison of calculated and measured pressures.
Advantage: Time saving
Disadvantage: Low accuracy (compared to ex-situ calibration)
Accuracy of volume measurement and Baratron calibration
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Calibrating RGA vs Extractor gauge by gas injection
• Pressure in RGA port and the gauge port should be the same due to test chamber symmetry
• Pressure in the test chamber varied between ~10-10 mbar to 10-6 mbar.
• Pressure recorded for the gauge and RGA peaks
• At 10-10 mbar to 10-8 mbar the injected gas might be not dominant in gas spectrum
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NEG-1 rig test chamber
RGA
Extractor
Gauge Gas
injection
Pumping
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Analysis of the measurements
RGA spectrum: there is always a mixture of gases
Assuming that the calibration coefficients ai for the gauge and
bi for the RGA, the measured currents are:
Then
Details of calibration coefficient matrix calculation was
covered by B. Jenninger in his talk in Session 2 and
in our paper in J. Vac. Sci. Technol. A 26 (2008), p. 1474.
( ) ; ( )g i i RGA i iI i a P I i b P
( )ig i i RGA
i i i
aI a P I i
b
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Typical results
Gas Type H2 CH4 CO N2 Ar CO2
Gauge
ai /aN2
0.44 1.5 1.04 1 1.3 1.6
RGA-FAR
bi /bN2
2.5 1.4 1.05 1 1.3 1.4
RGA-SEM
bi /bN2
4 - 7 1.7 1.05 1 1.3 1.4
The RGA calibration coefficients are normalised to a Nitrogen
coefficient and compared with referenced coefficients for an
ionisation gauge
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RGA calibration check vs. extractor gauge
15/40 WS-63, 14-19 September 2010, Ávila, Spain
Injection: H2 CO CO2 CH4 H2 CO CO2 CH4
0 100 200 300 4001 10
10
1 109
1 108
1 107
1 106
1 105
Pext measured
Pext calculated
H2
mass15
mass 16
CO
Ar
CO2
mass 12
mass 14
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Calibration of Extractor Gauge and RGA‟s
Compared to our
measurements
H2 differs by a factor of 4.3
CO differs by a factor of 2.4
CH4 differs by a factor of 3.0
CO2 differs by a factor of 4.7
Ar differs by a factor of 3.3
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Probability of
Ionisation for ionisation
gauges
H2 = 0.44
CO = 1.05
CH4 = 1.6
CO2 = 1.4
Ar = 1.2
Linearity over pressure ranges
2 RGAs and extractor gauge measured simultaneously
Injection of H2, CH4, CO and CO2
Relative sensitivity normalised to one measured at 10-7 mbar
+5% -10% -10% +60%
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SEM vs. FAR
Calibration with a Faraday cap is quite stable – no drift over
~3 years detected.
SEM is calibrated against a Faraday cap at P > 10-10 mbar
Checked during each experiment when both SEM and FAR
used
Residual sensitivity coefficients need to be corrected every
~3 months
WS-63, 14-19 September 2010, Ávila, Spain 18
Electron Stimulated Desorption (ESD)
Another factor to consider with RGA data at low pressure is
the influence of ESD from the ion source.
Typical ESD generated peaks include:
H+, O+, F+, 35Cl+ and 37Cl+
If unaccounted for it can lead to false conclusions in
interpretation of RGA data.
This is particularly important when considering the influence
of Oxygen containing species when activating GaAs
photocathodes. These species are considered a
contaminant and can „kill‟ the QE of a GaAs surface.
Suggestions are that partial pressures of < 10-14 mbar for
such species is required.
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Influence of ESD Peaks in RGA Data
10-13
10-12
10-11
10-10
10-9
0 20 40 60 80 100
RGA Scan from Outgassing System
Pa
rtia
l P
ressu
re (
mb
ar)
Mass
10-13
10-12
10-11
10-10
10-9
0 20 40 60 80 100
RGA Scan from Outgassing System
Pa
rtia
l P
ressu
re (
mbar)
Mass (amu)
ESD Peaks only
Gas phase and ESD species have different energies which allow separation between
the two.
Mass 19 is the dominant peak
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10-13
10-12
10-11
10-10
10-9
0 20 40 60 80 100
RGA Scan showing the influence of ESD peaks.P
artia
l Pre
ssur
e (m
bar)
Mass (amu)
GaAs photocathode studies
0
5E-13
1E-12
1.5E-12
2E-12
2.5E-12
3E-12
0 10 20 30 40 50 60
Before injection
RGA scan showing ideal vacuum system
P [
mb
ar]
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GaAs lifetime studies, purposely poisoning the cathode
0 500 1000 1500 2000 25000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
QE
Pressure
Elapsed time (s)
QE
(a
.u.)
5.0x10-11
1.0x10-10
1.5x10-10
2.0x10-10
2.5x10-10
3.0x10-10
Pre
ss
ure
(m
bar)
0 500 1000 1500 2000 25000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
QE
Pressure
Elapsed time (s)
QE
(a
.u.)
5.0x10-11
1.0x10-10
1.5x10-10
2.0x10-10
2.5x10-10
3.0x10-10
Pre
ss
ure
(m
bar)
0 500 1000 1500 2000 25000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
O2 exposure
CO exposure
CO2 exposure
QE
Pressure
Elapsed time (s)
QE
(a
.u.)
5.0x10-11
1.0x10-10
1.5x10-10
2.0x10-10
2.5x10-10
3.0x10-10
Pre
ss
ure
(m
bar)
-5E-13
0
5E-13
1E-12
1.5E-12
2E-12
2.5E-12
0 10 20 30 40 50
-1E-12
0
1E-12
2E-12
3E-12
4E-12
5E-12
6E-12
7E-12
-10 10 30 50
-2E-12
0
2E-12
4E-12
6E-12
8E-12
-10 0 10 20 30 40 50
CO
injection
CO2
injection
O2
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Conclusions
Vacuum science requires:
a quantitative RGA:
Pi = f (Ii) = Ii /C(Ii,Ptot,Ie,Eion,...)
Stabile, including XHV
With high traceability
Low outgassing
In-situ RGA calibration can be performed against a total
pressure UHV/XHV gauge when gas injection is available
In-situ RGA calibration check might be performed without a
gas injection
Ex-situ RGA calibration would bring more confidence in the
RGA performance
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